Knowledge base
A large turbocharged engine does not have one cooling circuit but two. The high-temperature (HT) circuit carries the heat of the cylinder jackets and heads at a high temperature; the low-temperature (LT) circuit cools the charge air after the turbocharger, and often the oil, at a much lower one. An LT/HT radiator rejects both to the ambient air in one unit. This guide explains why the circuits are separate, how the two coils are arranged, and why the LT circuit is usually the one that decides the size of the radiator.
Why An Engine Needs Two Temperatures
The jacket water leaves the engine hot, because a hot block runs efficiently and the engine maker wants the metal at a steady temperature. The charge air is a different matter: the turbocharger compresses and heats it, and every degree it is cooled back raises its density, which means more air and more power in the cylinder and lower nitrogen oxides. The charge air cooler therefore needs coolant that is as cold as the radiator can make it.
Mixing the two in one loop would either overheat the charge air or overcool the jackets. So the engine has two loops, each with its own pump, its own flow and temperature limits and its own allowable pressure drop; the engine maker states them on the engine data sheet.
One Casing, Two Coils
An LT/HT radiator holds two separate coils with their own collectors and connections, so the two circuits never mix. The usual arrangement puts the coils in series in the air stream, with the LT coil on the air-inlet side: the coolest air meets the circuit that needs it, and the air, already warmed by the LT coil, then passes the HT coil, which works at a much higher temperature and does not mind. The alternative is to put the coils side by side so that both see ambient air; it makes the casing wider but gives the LT circuit a lower air temperature to work against.
Both coils share one fan group and one casing, which saves footprint, frame and cabling compared with two separate radiators. Where the site, the transport or the layout asks for it, two separate radiators remain a sound choice; the engine circuits are the same either way.
Why The LT Circuit Decides The Size
The heat a radiator can reject depends on the temperature difference between the coolant and the ambient air. The HT circuit works with a generous difference. The LT circuit has to deliver coolant to the charge air cooler that is only a little warmer than the air, and on a hot day or at altitude that difference shrinks further. A small difference means a large coil surface and a high air flow for the same duty, so in hot climates the LT coil is often the larger part of the unit.
This is also why the ambient temperature must be stated honestly in an enquiry: a few degrees added to the design ambient can change the LT coil more than any other input. Engine makers give an LT circuit limit at a stated ambient; a radiator designed for a lower ambient than the site will see the limit exceeded on the hottest days.
What The Design Has To Match
For each circuit the radiator is designed to the heat duty, the flow, the inlet and outlet temperatures, the allowed coolant pressure drop and the glycol mixture; the coolant side is checked so that the flow velocity keeps the pressure drop within the engine maker's limit without making the tubes noisy or eroded. On the air side the fan group is chosen for the combined air flow and the sound limit, with electric fans (AC or EC, speed controlled) for remote units. The thermal design follows the engine data sheet, and the unit is built to the pressure and corrosion class of the site.
TECS Engineering designs and builds LT/HT radiators for gas and diesel generator sets and engine power plants, as horizontal and vertical remote units, V-type units and, for smaller sets, mechanical driven units. Send the engine data sheet or the two duties, the site ambient and the installation limits, and the proposal shows both circuits.
In Short
- HT circuit: engine jackets; LT circuit: charge air and often the oil
- Two coils with separate collectors in one casing and under one fan group
- LT coil usually on the air-inlet side, so it meets the coolest air
- The small LT temperature difference often makes it the larger coil in hot climates
- Each circuit designed to its own flow, temperatures and pressure drop
- Horizontal, vertical, V-type or mechanical driven, with AC or EC fans
What To Send With An Enquiry
- Engine data sheet, or heat duty, flow and temperatures for both circuits
- Allowed coolant pressure drop for each circuit and the glycol mixture
- Site ambient (the hottest day the engine must carry full load), altitude, humidity
- Installation: remote horizontal, vertical, V-type, or engine mounted
- Sound limit and the measurement distance
- Power supply for the fans, corrosion class and space limits
Questions We Are Often Asked
What does LT and HT stand for?
Low temperature and high temperature. The HT circuit cools the engine jackets, the LT circuit cools the charge air and often the oil. They run at different temperatures and are kept separate.
Can one radiator serve both circuits?
Yes. An LT/HT radiator has two coils in one casing, each with its own connections, under one fan group. The circuits stay separate.
Which circuit is the harder one to cool?
The LT circuit, because its coolant must stay close to the ambient air temperature. A small temperature difference needs a large coil surface and a high air flow.
Is an LT/HT radiator bigger than a single-circuit one?
For the same engine it carries both duties, so it is larger than a radiator for the HT duty alone, but smaller and simpler than two separate units with their own frames and fans.
What happens if the real ambient is hotter than the design ambient?
The LT coolant temperature rises above the engine maker's limit first, the charge air gets hotter and the engine derates or alarms. The design ambient should be the hottest the site will see at full load.
Contact Us
Send us the heat duty, the fluid temperatures and flow rates, the ambient conditions and the installation limits. We answer with a design proposal - or write to us on WhatsApp.
